Enumeration of Salmonella? Help Understanding Salmonella Methodology

Hello, I've been tasked with obtaining literature regarding Salmonella in certain foods. I'm trying to find information on the number of Salmonella cells in a food item.... but I can only find literature regarding the detection of Salmonella in a food (e.g. percentage of samples that test positve) and not the actual enumeration! However, I often find studies showing detection and enumeration of other microorganisns, e.g. E. coli, B. cereus. I also briefly looked at the FDA BAM, and there's a chapter on Salmonella but it only goes into isolation and identification of Salmonella. There's another chapter on the isolation, identification, and enumeration of E. coli and coliform bacteria.

I'm not a microbiologist... so is there a reason for this? Is Salmonella normally not enumerated? Why? I've looked at lots of scientific articles... are my chances slim for finding information on Salmonella enumeration?!  

Any help is appreciated. 

Thank you! 

A quick Google search provided a good article at this link: 

https://aem.asm.org/content/74/5/1299

"Moreover, the infection dose of Salmonella depends on the food item itself."  

IME Salmonella is a difficult culture, and the mere presence of Salmonella at ANY level requires action, so no need to spend extra dollars counting colonies.

Thank you for the link! I also saw that article. I agree that any presence of Salmonella requires action, but I want to research how much pathogens are present general in certain food items so we know how much log reduction is appropriate. I know 5 log is standard but is that always appropriate? Will we need 6 log? Or is something lower adequate? E.g. Almond Board recommends at least 4 log for almonds! 

Unfortunately the data for numeration of Salmonella in foods is low <-- but there are many articles that state the incident rate or percentage of Salmonella in samples! 

Thank you for the link! I also saw that article. I agree that any presence of Salmonella requires action, but I want to research how much pathogens are present general in certain food items so we know how much log reduction is appropriate. I know 5 log is standard but is that always appropriate? Will we need 6 log? Or is something lower adequate? E.g. Almond Board recommends at least 4 log for almonds! 

 

Unfortunately the data for numeration of Salmonella in foods is low <-- but there are many articles that state the incident rate or percentage of Salmonella in samples! 

Hi zechaplz,

IIRC the quantitative logic/typical contamination assumptions which resulted in the various reduction log values such as you mention has mostly been published in the original official documentation.

I previously attached here (somewhere) a few related papers however the data  did indeed  require some significant searching. This general topic is intrinsic to setting Regulatory microbiological standards and specifications and is also discussed at length in various Codex publications and certain Volumes of the well-known Microbiology in Foods  Book Series.

The reason why most methodologies for Salmonella focus on detection only is simple - it is afaik Globally set as a zero-tolerant species in RTE food and often in NRTE foods also. The usual reasoning is based on the fact that certain species/groups of microbiological pathogens are considered to represent a  severe health risk when present in very low numbers in foods (2 examples are pathogenic E.coli species and some Salmonella species).

Perhaps somewhat remarkably, afaik, all the 2000+ species of Salmonella are considered pathogenic albeit with varying degrees of "sensitivity". Consequently officialdom implemented a worst case scenario as the usual standard for RTE items. NRTE items can be a more complex issue.

Dear Zech,

Enumeration is not necessary, when the identification of the species can already be done without it. After the analysis you need to destroy all pathogen or risky cultures, so the more you enumerate, the more you need to destroy.

It's also for safety reasons for the laboratory technicians and other people working around, to keep the numbers of such dangerous bacteria as low as possible. 

Kind regards,

Gerard Heerkens

Dear Zech,

 

Enumeration is not necessary, when the identification of the species can already be done without it. After the analysis you need to destroy all pathogen or risky cultures, so the more you enumerate, the more you need to destroy.

 

It's also for safety reasons for the laboratory technicians and other people working around, to keep the numbers of such dangerous bacteria as low as possible. 

 

Kind regards,

 

Gerard Heerkens

Hi Gerard,

I agree yr comment on maintaining safety for food/environment/personnel.

A few additional thoughts -

(1) Different species of pathogenic microbiological bacteria are associated with varying degrees of health risk. And similarly for implemented, related Laboratories.

(2) Some Regulatory requirements necessitate enumeration of certain (non-zero tolerant) species of pathogenic bacteria.

(3) Laboratories working with any type of microbiological species must implement appropriate precautions.

(4) It is sometimes useful to quantitate a level of contamination.

With respect to the OP's specific query, these documents may assist -

 FSIS - Understanding Microbiological Sampling and Testing,2016.pdf   1.15MB   34 downloads

 Development of FDA Salmonella Sampling and Testing Plans.pdf   282.38KB   26 downloads

(it's 2nd, classic, informative, highly detailed, reference is fully accessible as a "Google book" although media etc now in use have progressed with time)

Hi Charles,

Thank you for this additional information, completing the picture.

Kind regards,

Gerard

Hi,

as written before - for Salmonella in most cases a zero tolerance. exist.

One additional thought acc FDA sampling e.g. for chocolate  n=30 oder n=60

n=30 c m M and 25 g would mean for enumeration -> 30 tests -> 30 results -> costs for 30 tests

n=30 c=0 and 25 g means creating a combined sample of 750 g in x litres growth medium -> 1 result (growth Y/N)

Rgds

moskito

Hi, Salmonella should not be deteced in 25 CFU/g according Australian Standard requirement in Fresh Produce ( Leaf and Veg). 

Hi, Salmonella should not be deteced in 25 CFU/g according Australian Standard requirement in Fresh Produce ( Leaf and Veg). 

I assume you mean not detected in a 25g sample. I anticipate there is also a Sampling Plan. as per Post 8. Unless the Australian Sample Size = 1 ?

I assume you mean not detected in a 25g sample. I anticipate there is also a Sampling Plan. as per Post 8. Unless the Australian Sample Size = 1 ?

Listeria should not be detected in 25g sample size.
1. Presumptive listeria.
Wait for 2-3 days
2. Listeria confirmed or not
3. Listeria confirmed
Wait for 2 days
4. Listeria ID and count
5. If it is a Listeria Mono, we wait for a count and we are hoping that is less than 10 cfu/g.
Or
6. Not L. mono - for example, it is L. Seeligeri with count.

Our major retailers in australia want to see Listeria ND in 25g and we do not need to recall if L. Mono is less than 10cfu/g.




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